While they’re still working to pinpoint the source, workers have made progress in keeping an oil spill north of Denverfrom getting any worse:

Federal officials say a trench dug to block toxic chemicals from seeping from a refinery into a creek is working.

Environmental Protection Agency spokesman Matthew Allen said Monday a 240-foot trench completed over the weekend is preventing a gasoline-like substance from seeping from the Suncor Energy refinery into Sand Creek and the South Platte River.

Crews over the weekend worked in subfreezing temperatures cleaning up the spill, which may have been caused by a leaking pipe.

A previous operator of the refinery, Conoco, in 1993 was ordered by a federal judge to “take all measures necessary to terminate the discharge of all pollutants into Sand Creek from the refinery property.”

The refinery was last penalized by the state Hazardous Materials and Waste Management Division in 1998, when Conoco was fined $72,500 after cancer-causing benzene was discovered in Sand Creek. The refinery also was ordered to clean up sulfur emissions.

In 2009, the U.S. Department of Labor’s Occupational Safety and Health Administration cited the facility for workplace safety and health violations, proposing $130,500 in penalties.

And now the latest spill, which comes just weeks after the State Department delayed a decision on the Keystone XL tar sands pipeline proposed by TransCanada. That pipeline would carry up to 900,000 barrels per day of tar sands oil from Canada to refineries on America’s Gulf Coast. The pipeline would cut through six American heartland states, including Montana, South Dakota, Nebraska, Kansas, Oklahoma and Texas.